Automatic degaussing apparatus for minimizing residual current during steady state operation

ABSTRACT

A degaussing coil structure is connected in series with a temperature dependent resistor and coupled to a source of alternating potential. A frequency dependent impedance is connected in series with the temperature dependent resistor and coupled to another source of alternating potential. When the temperature dependent resistor exhibits its steady state operating resistance, a voltage is developed at the junction of the temperature dependent resistor and degaussing coil structure which reduces the voltage gradient across the coil structure to reduce the residual current flowing through the structure.

United States Patent 3,699,400 Marsh, Jr. 1 Oct. 17, 1972 [54] AUTOMATIC DEGAUSSING 7 3,582,721 6/1971 Van Hoorn ..3l7/l57.5

' APPARATUS FOR MINIMIZING RESIDUAL CURRENT DURING Primary Examiner-J. D. Miller STEADY STATE OPERATION Assistant Examiner-Harry E. Moose, Jr. [72] Inventor: James Courtland Marsh, Jr., Indian- Att0mey Eugene whltacre 57 ABSTRACT [73] Assignee: RCA Corporation v v A degaussmg coll structure is connected in series with Filedi Ma 1971- a temperature dependent resistor and coupled to a [21] APPL d 121,666 source of alternating potential. A frequency dependent impedance is connected in series with the temperature dependent resistor and coupled to another [52] 0.8. CI ..317/157.5, 317/132 source of alternating potential, w the temperature [51] lltt. Cl. ..H0lll 47/00 dependent resistor exhibits its steady State operating [58] Field Of Search .L..3 l7/l57.5; 315/8 resistance a voltage is developed at the junction of the temperature dependent resistor and degaussing [56] References C'ted coil structure which reduces the voltage gradient UNITED STATES PATENTS across the coil structure to reduce the residual current flowing through the structure. 3,492,543 l/l970 Muranaga ..317/157.5 3,344,307 9/1967 Van Anrooy ..3 l 7/157.5 11 Claims, 6 Drawing Figures PATENTEDHBT 11 I 12 3.699400 sum 1 0F 2 ,Vil/

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a/Kw Alto/nay 1 AUTOMATIC DEGAUSSING APPARATUS FOR The present invention relates to automatic,

degaussing circuits and more particularly, to a circuit for minimizing the residual current flowing through the degaussing coils during steady state operation.

In a shadow mask type color picture tube, the metal mask and its supporting structure together with other metallic parts of the tube are subject to being mag netized in various ways such as in shipment to and continued use in a consumers home. Such undesired magnetization is caused by bringing the picture tube into proximity with magnetizing fields associated with structures such as trucks, elevators and the like, and also by exposure during use to influences such as the earths magnetic field. Such random magnetization often adversely affects-the performance of a color television receiver in which the picture tube is embodied.

The remedy for such magnetization is the degaussing of the apparatus. Often, automatic degaussing of a color television picture tube is provided which usually is accomplished by providing a coil structure suitably placed relative to the'picturetube so that the field produced thereby encompasses the shadow mask and its supporting structure in the picture tube. Such a coil structure is energized by alternating current which initially has a substantial magnitude but which subsequently is gradually diminished to a relatively low magnitude residual current. Many circuits for energizing such coil structures employ components such as thermistors and varistors to cause the current flowing through the coil structure to diminish and to effectively disconnect the coil structure from the source of alternating current after the energizing current reaches its relatively low magnitude residual current.

A problem is encountered in many circuits in that the low magnitude residual current is often still sufficient to degrade the picture reproduced on the face of the kinescope. While the specific degradation of the picture is dependent upon the magnitude of the residual current and the physical dimensions of the coil structure and its location in the receiver, one type of degradation which can be encountered is a series of thin horizontal lines which slowly move in a vertical direction across the face of the kinescope during a color broadcast.

One prior art technique for minimizing the residual current flowing through the degaussing coil structure is to place a thermistor in series with the degaussing coil structure across a source of alternating potential with the junction of the thermistor and coil structure connected by a resistor to another source of alternating potential. The component values and operating voltages are selected so that when steady state operation is achieved (the thermistor is heated to its operating temperature), the voltage division between the thermistor and resistor results in substantially equal voltages being impressed on both ends of the degaussing coil structure. This reduces the voltage gradient across the degaussing coil structure and, hence, the residual current flow through the coil structure. While this technique helps in'diminishing the residual current,

spikes of current can still occur where the alternating voltage applied across the thermistor and degaussing coil structure contain high frequency components.

High frequency components are often present in the secondary winding circuit of the power transformer of television receivers even when the primary winding is energized by a pure sine waveform voltage. This is because of loading of the secondary winding by circuit components, especially diodes, which when they are biased into and out of conduction, cause abrupt changes in the secondary winding voltage waveform and, thus, introduce high frequency components. This is especially troublesome in cold chassis type television receivers; that is, television receivers employing an isolation transformer. In such receivers, to minimize the possibility of shock hazards, automatic degaussing circuits are connected in the secondary winding side of the television receivers power transformer where high frequency components can be present.

It is an object of the present invention to minimize the residual current flowing through the degaussing coil structure of a television receiver during steady state operation.

In accordance with the present invention, a degaussing coil structure is electrically connected in series with a temperature dependent resistor and coupled to a source of alternating potential. A frequency dependent impedance is connected in series with the temperature dependent resistor and coupled to another source of alternating potential. When the temperature dependent resistor exhibits its steady state operating resistance, a voltage is developed at the junction of the temperature dependent resistor and degaussing coil structure which reduces the voltage gradient across the degaussing coil structure to reduce the residual current flowing through the structure.

A complete understanding of the invention may be obtained from the following detailed description, when taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic circuit diagram of an automatic degaussing apparatus embodying the present invention; and

FIGS. 2a, 2b, 2c, 2d and 2e are a series of curves depicting waveforms helpful in an understanding of the ry winding 16 of an iron core transformer 20. The

transformer 20 may be the power transformer of a television receiver 22. The voltage developed across the secondary winding 24 of the transformer 20 is applied to a half wave rectifier circuit 27 to develop a +225 volt DC potential for use in the television receiver 22. The secondary winding 24 includes a tap 26 which is connected, as is the end 28 of the secondary winding 24, to a full wave bridge rectifier circuit 30 to develop a volt DC potential for use in the television receiver 22.

A degaussing coil 32 is connected in series with a positive coefficient temperature dependent resistor 34 between the transformer secondary winding tap 26 and end 28. Positive coefficient temperature dependent resistors are thermistors whose resistance increases with increased temperature which occurs when current flows through the component. The junction 36 of the positive coefficient temperature dependent resistor 34 and degaussing coil 32 is connected by the parallel similar shaped distorted sinusoidal waveform voltage of greater magnitude is developed between the ends 2842 of the secondary winding 24. Since the positive coefficient temperature dependent resistor 34 is initially cold (assuming switch 14, before closing, had been open a sufficient period of time), a relativelylarge current flows between the tap 26 and the end 28 of the secondary winding of the transformer through the degaussing coil 32 and series connected positive coefficient temperature dependent resistor 34. Additionally, a current flowsbetween ends 42 and 28 of the secondary winding of the transformer through the series combination of the positive coefficient temperature dependent resistor 34 and parallel connected resistor 38- capacitor 40.

As current flows through the positive coefficient temperature dependent resistor 34, the resistor begins to heat up and display an increasing resistance. The increasing resistance of the positive coefficienttemperature dependent resistor 34 causes the current flowing between the tap 26 and end 28 of the secondary winding of the transformer to diminish. Consequently, the magnetic field produced by the degaussing coil 32 also begins to diminish providing a degaussing effect on the television receiver kinescope shadow mask and supporting structure, not shown. I,

As the resistance of the positive coefficient temperature dependent resistor 34 increases, the voltage at the junction 36 begins to rise due to the voltage division between the capacitor 40 and the resistors 38 and 34. This reduces the voltage gradient across the degaussing coil 32 between the tap 26 of the secondary winding of the transformer and the junction 36. As a result, the current flowing through the degaussing coil 36 reduces. When the positive coefficient temperature dependent resistor 34 achieves its steady state operating temperature and resistance, substantially equal voltages are present at the tap 26 of the secondary winding of the transformer and the junction 36 which results in a very low residual current flowing through the degaussing coil 32.

As is apparent in FIG. 2b, the voltage developed between the tap 26 and end 28 of the secondary winding 24 of the transformer is a distorted sinusoidal waveform voltage. It will be noted that the peaks 44 of the waveform are substantially flat and that the initial excursions 46 from the peaks 44 toward the abscissa are substantially vertical. These distortions are associated with the loading of the secondary winding 24 by circuit components associated with the television receiver 22. The flat portions 44 are associated with conduction of the diodes of the half wave rectifier circuit 27 and full wave bridge rectifier circuit 30, while the vertical portions 46 of the waveform are associated with the turn-off of the diodes.

The distortion of the voltage developed in the secondary winding 24 of the transformer 20 from a pure 4 sinusoidal waveform introduces high frequency signal components. The high frequency signal components introduced in the secondary winding circuit of the transformer 20 find a low impedance path through the shunt capacitance of the positive coefficient temperature dependent resistor 34. This particularly clear in FIG. 20, which is a plot of the residual current flowing through the degaussing coil 32 during steady state operation with theresistor 38 and capacitor 40 removed from the circuit. The residual current has a distorted sinusoidal waveform shape with current spikes 48. These current spikes are attributable to the flow of the high frequency signal components through the degaussing coil 32 and the shunt capacitance of the positive coefficient temperature dependent resistor 34.

When resistor 38 (but not capacitor 40) is included in the circuit, a diminished residual current flows through the degaussing coil due to a reduced fundamental frequency signal component gradient across the coil. This is shown in FIG. 2d, which is a plot of the residual current flowing through the degaussing coil 32 during steady state operation with resistor 38 (but not capacitor 40) connected in the circuit. The shunt capacitance of the positive coefficient temperature dependent resistor 34, however, provides a low impedance path for the high frequency signal components, and a voltage gradient of these high frequency signal components remains across the degaussing coil 32.

Since the positive coefficient temperature dependent resistor 34 has a voltage dependent characteristic, the resistance exhibited by the temperature dependent resistor changes as the voltage at the junction 36 changes. Consequently, the voltage division between the resistor 38 and the positive coefficient temperature dependent resistor 34 varies, introducing harmonic frequency voltage components across the temperature dependent resistor 34 which are not present at the tap 26 on the secondary winding of the transformer 20. As a result, a voltage gradient exists across the degaussing coil 32 causing a residual current flow even where the high frequency signal components are not present. Nevertheless, the magnitude of the residual current attributable to the harmonic frequency voltage components gradient across the degaussing coil 32 is not sufficient to be objectionable. The harmonic frequency voltage components, however, combine with the high frequency signal components to produce a residual current waveform (FIG. 2d) having a shape and zero current cross over points which differ from the waveform shown in FIG. 20. The harmonic frequency voltage components and the high frequency signal components, in the absence of the fundamental frequency signal component, combine in a manner to create a voltage gradient across the degaussing coil 32 where the magnitude of the residual current spikes 50 are increased. The spikes 50 of the residual current are of a sufficient magnitude to cause a degraded picture to appear on the face of the kinescope. The degradation may be in the form of the previously described thin horizontal lines which slowly travel in a vertical direction across the face of the color kinescope.

By including the capacitor 40 in conjunction with the resistor 38 in the circuit, the spikes of residual current can be substantially reduced. The waveform of current pendent resistor 34. The capacitor 40 permits a greater magnitude of the high frequency signal components to be coupled to the junction 36 which reduces the voltage gradient of high frequency signal components across thedegaussing coil 32 to decrease the magnitude of the residual current spikes flowing through the degaussing coil.

What is claimed is:

1. An automatic degaussing circuit for electrical apparatus comprising:

a first source of alternating potential;

a second source of alternating potential;

said electrical apparatus including a load circuit which introduces high frequency components into the voltage waveforms of said first and said second sources of alternating potential;

a degaussing coil structure and a temperature dependent resistor connected in series and coupled to said first source of alternating potential, said temperature dependent resistor having a shunt capacitance which provides a low impedance path for current flow due to said high frequency voltage components; and

a frequency dependent impedance having a resistive component, said frequency dependent impedance connected in series with said temperature depen dent resistance and coupled to said second source of alternating potential such that when said temperature dependent resistor exhibits its steady state operating resistance, a voltage is developed at the junction of said temperature dependent resistor and said degaussing coil structure which reduces the voltage gradient across said degaussing coil due to low and high frequency voltage components to reduce the residual current flowing through said degaussing coil.

2. A degaussing circuit for electrical apparatus as defined in claim 1 wherein said temperature dependent resistor is a positive coefficient temperature dependent resistor.

3. A degaussing circuit for electrical apparatus as defined in claim 2 wherein said frequency dependent impedance includes a capacitor.

4. A degaussing circuit for electrical apparatus as defined in claim 2 wherein said frequency dependent impedance includes a parallel connected resistor and capacitor.

5. A degaussing circuit for electrical apparatus as defined in claim 1 wherein the voltage provided by said first and said second source of alternating potential each has a distorted sinusoidal waveform.

6. A degaussing circuit for electrical apparatus comprising:

a transformer having a primary winding coupled through a switch to a source of alternating potential having a sinusoidal waveform and a secondary winding having a first and a second end and a tap;

a load circuit coupled to said secondary winding which introduces high frequency components into the voltage waveform in said secondary winding;

a temperature dependent resistor having a shunt capacitance and a frequency dependent impedance having a resistive component, said temperature dependent resistor and said frequency dependent impedance connected in series between the first and second end of the seco dary winding of said transformer; and

a degaussing coil structure coupled between the tap connection of the secondary winding of said transformer and the junction of said frequency dependent impedance and said temperature dependent resistor.

7. A degaussing circuit for electrical apparatus as defined in claim 6 wherein said temperature dependent resistor is a positive coefficient temperature dependent resistor.

8. A degaussing circuit for electrical apparatus as defined in claim 6 wherein said electrical apparatus is a television receiver, and said load circuit coupled to the secondary winding of said transformer is a rectifier circuit for providing a rectified voltage for said television receiver.

9. A degaussing circuit for electrical apparatus as defined in claim 8 wherein said temperature dependent resistor is a positive coefficient temperature dependent resistor.

10. A degaussing circuit for electrical apparatus as defined in claim 9 wherein said frequency dependent impedance includes a capacitor.

11. A degaussing circuit for electrical apparatus as defined in claim 9 wherein said frequency dependent impedance includes a parallel connected resistor and a capacitor. 

1. An automatic degaussing circuit for electrical apparatus comprising: a first source of alternating potential; a second source of alternating potential; said electrical apparatus including a load circuit which introduces high frequency components into the voltage waveforms of said first and said second sources of alternating potential; a degaussing coil structure and a temperature dependent resistor connectEd in series and coupled to said first source of alternating potential, said temperature dependent resistor having a shunt capacitance which provides a low impedance path for current flow due to said high frequency voltage components; and a frequency dependent impedance having a resistive component, said frequency dependent impedance connected in series with said temperature dependent resistance and coupled to said second source of alternating potential such that when said temperature dependent resistor exhibits its steady state operating resistance, a voltage is developed at the junction of said temperature dependent resistor and said degaussing coil structure which reduces the voltage gradient across said degaussing coil due to low and high frequency voltage components to reduce the residual current flowing through said degaussing coil.
 2. A degaussing circuit for electrical apparatus as defined in claim 1 wherein said temperature dependent resistor is a positive coefficient temperature dependent resistor.
 3. A degaussing circuit for electrical apparatus as defined in claim 2 wherein said frequency dependent impedance includes a capacitor.
 4. A degaussing circuit for electrical apparatus as defined in claim 2 wherein said frequency dependent impedance includes a parallel connected resistor and capacitor.
 5. A degaussing circuit for electrical apparatus as defined in claim 1 wherein the voltage provided by said first and said second source of alternating potential each has a distorted sinusoidal waveform.
 6. A degaussing circuit for electrical apparatus comprising: a transformer having a primary winding coupled through a switch to a source of alternating potential having a sinusoidal waveform and a secondary winding having a first and a second end and a tap; a load circuit coupled to said secondary winding which introduces high frequency components into the voltage waveform in said secondary winding; a temperature dependent resistor having a shunt capacitance and a frequency dependent impedance having a resistive component, said temperature dependent resistor and said frequency dependent impedance connected in series between the first and second end of the secondary winding of said transformer; and a degaussing coil structure coupled between the tap connection of the secondary winding of said transformer and the junction of said frequency dependent impedance and said temperature dependent resistor.
 7. A degaussing circuit for electrical apparatus as defined in claim 6 wherein said temperature dependent resistor is a positive coefficient temperature dependent resistor.
 8. A degaussing circuit for electrical apparatus as defined in claim 6 wherein said electrical apparatus is a television receiver, and said load circuit coupled to the secondary winding of said transformer is a rectifier circuit for providing a rectified voltage for said television receiver.
 9. A degaussing circuit for electrical apparatus as defined in claim 8 wherein said temperature dependent resistor is a positive coefficient temperature dependent resistor.
 10. A degaussing circuit for electrical apparatus as defined in claim 9 wherein said frequency dependent impedance includes a capacitor.
 11. A degaussing circuit for electrical apparatus as defined in claim 9 wherein said frequency dependent impedance includes a parallel connected resistor and a capacitor. 